This project isn't as straight forward as you'd like if what you want is the 'look' of a professional job. Wood stain, by nature, will penetrate into the wood and is not easily sanded out. Essentially, to get the look of a professional job, you'd have to replace the trim. Most trim these days, has lots of fancy-dancy profiles to them. This makes them impossible to 'clean up' in the aforementioned way. (And not worth the $$$.)
If your scratches and blemishes are many and profound, replace the trim (IMO). If they're slight and spotty, you need to find a matching stain. Once you apply the stain, you'll need to sand the entire trim piece and add a couple of layers of polyurethane. You can bring in a piece of the trim into the lumber yard or Home Center for advice on how to 'fix' up your trim.
You really don't restain the trim in the house, per se, because you'll never get it right (as I mentioned initially of how the wood will absorbe the stain.) That is, unless your trim is completely flat! Usually, you touch up the trim, sand it with a light sand paper (or liquid sandpaper), and re-poly. If you can get a hold of the builder, maybe he can tell you what stain they used.